This application makes reference to and claims all benefits accruing under 35 U.S.C. Section 119 from an application entitled xe2x80x9cDEVICE FOR FABRICATING POLARIZATION INSENSITIVE LONG PERIOD FIBER GRATINGxe2x80x9d filed in the Korean Industrial Property Office on Aug. 4, 2000 and there duly assigned Serial No. 2000-45196.
1. Field of the Invention
The present invention relates generally to an optical communication, and in particular to a device for fabricating a long-period fiber grating used in an optical communication field.
2. Description of the Related Art
Long-period fiber grating devices selectively remove light at specific wavelengths by mode conversion. Long-period gratings remove light without reflection by converting it from a guided mode to a non-guided mode. A non-guided mode is a mode which is not confined to the core, but rather, is defined by the entire waveguide structure, i.e., a cladding mode. The long-period fiber grating has been used as a fiber filter, such as a non-reflecting band rejection filter and band-pass filter, an optical sensor, and a gain flattener for the erbium-doped amplifier.
A typical method for fabricating the long-period fiber grating is based on a well known principle that xe2x80x9cgermanium-doped optical fiber is photosensitive to ultraviolet lightxe2x80x9d in pp.647-649 of APPL. PHYS. Lett. Vol. 32, xe2x80x9cPhotosensitivity in optical fiber wave guides: Application to reflection filter fabricationxe2x80x9d suggested by K. O. Hill et. al. and published in 1978.
A representative method for fabricating the long-period fiber grating is ultraviolet irradiation using xe2x80x9cside writingxe2x80x9d as suggested in pp.823-825 of Opt. Lett. Vol. 14, xe2x80x9cFormation of Bragg gratings in optical fibers by transverse holographic methodxe2x80x9d by G. Meltz et. al. and published in 1989.
FIG. 1 is a structure diagram illustrating a conventional device for fabricating a long-period fiber grating. Referring to FIG. 1, the conventional device 100 for fabricating the long-period fiber grating includes a ultraviolet laser 110 for irradiating a ultraviolet laser beam, a lens 120 for adjusting a focus of the ultraviolet laser beam irradiated from the ultraviolet laser 110, an amplitude mask 130 for selectively passing the ultraviolet laser beam from the lens 120, and a fixing unit 140 for fixing both ends of a fiber (f). The positioning and orientation of amplitude mask 130 are such that when ultraviolet laser beam is transmitted therethrough, an interference pattern is formed which extends through the fiber (f). Here, the period of the grating can be adjusted by tilting amplitude mask relative to the longitudinal extend of fiber (f).
When the conventional device 100, as described in the preceding paragraph, is used to fabricate the long-period fiber grating, the ultraviolet laser beam is asymmetrically irradiated on one side of the fiber, thus the refractive index of the fiber core is anisotropically varied due to a polarization status of the ultraviolet laser beam and causes birefringence in the fiber. That is, the core of the non-birefringent fiber is substantially circular-symmetric. The circular symmetry ensures that the refractive index of the core mode is essentially insensitive to the state of optical polarization. In contrast, in hi-birefringent single mode fibers the effective refractive index of the core mode is substantially different between two principal polarization states. For background information, the birefringence due to the polarization of the ultraviolet laser beam has been disclosed in detail in a treatise of T. Erdogan, p.2100 to p.2105 of xe2x80x98Journal of the optical society of America Bxe2x80x99 in 1994.
In contrast with conventional Bragg gratings, long-period gratings use a periodic spacing which is typically at least 10 times larger than the transmitted wavelength, and thus has serious variations of the refractive index due to the polarization status of the ultraviolet laser beam, namely high polarization dependence. The polarization dependence of the long-period fiber grating results in a undesirable PDL (Polarization Dependent Loss) and PMD (Polarization Mode Dispersion), thereby deteriorating the property of the long-period fiber grating as an optical communication device.
It is, therefore, the present invention relates to a device for fabricating a polarization insensitive long-period fiber grating, which can reduce the property loss due to birefringence that is resulting from the polarization status of a ultraviolet laser beam.
According to an aspect of the present invention to provide a device for fabricating a polarization insensitive long-period fiber grating, which can reduce a property loss due to birefringence that is generated when ultraviolet light is irradiated merely to one side of a fiber.
Accordingly, there is provided a device for fabricating a polarization insensitive long-period fiber grating. The device includes: an ultraviolet laser for irradiating a ultraviolet laser beam; an ultraviolet polarizer for converting a polarization status of the ultraviolet laser beam into a linear polarization status in parallel along the longitudinal direction of a fiber; a lens for adjusting a focus of the ultraviolet laser beam from the ultraviolet polarizer; an amplitude mask for selectively passing the ultraviolet laser beam from the lens; and a mount for holding both ends of the fiber such that the outer circumferential surface of the fiber can be exposed to the ultraviolet laser beam from the amplitude mask.
In another aspect of the present invention, a device for fabricating a polarization insensitive long period fiber grating includes: an ultraviolet laser for irradiating a ultraviolet laser beam; an ultraviolet polarizer for converting a polarization status of the ultraviolet laser beam into a linear polarization status in parallel along the longitudinal direction of a fiber; a lens for adjusting a focus of the ultraviolet laser beam from the ultraviolet polarizer; an amplitude mask for selectively passing the ultraviolet laser beam from the lens; a mount for holding both ends of the fiber such that the outer circumferential surface of the fiber can be exposed to the ultraviolet laser beam from the amplitude mask; and a mirror for reflecting the ultraviolet laser beam from the amplitude mask to the outer circumferential surface of the fiber.
According to another aspect of the invention, there is provided a method for passing light through an amplitude mask to create interference pattern into a fiber. The method includes the steps of: generating a ultraviolet laser beam; converting a polarization of the ultraviolet laser beam into a linear polarization in a parallel relationship with the fiber; directing the converted ultraviolet laser beam through an amplitude mask and into the fiber, such that when the converted ultraviolet laser beam is transmitted through the amplitude mask and exposed to along the fiber, an interference is formed which extends through the fiber.